Yes Nitrate above 40ppm is a problem. I'd do a careful 50% water change now and then about 25% each day until your nitrate is down close to the same as the tap water. Once it's there see if you can keep nitrate down with just a single 20% per week water change.
As for stocking levels, I have to say that none of us here believe a biorb 30 is anything other than a deathtrap for fish. It's simply not big enough to provide the stability and security a fish needs. However, technically speaking you could fit maybe 8" of small fish assuming : Adult length, slim bodies, omnivores, hardy and midwater swimmers.
This gives you room for 4 minnows. They should be kept in shoals of 6 or more though...
Just a quick update. Got the nitrate down to something closer to tap water. Will try another few changes to get it right down. Rotating a variety of foods which seem to be popular. The thing whatever it was seems to have gone, so it seems to have done the trick.